While tens of millions of people have abnormal, leak-prone sproutings of blood vessels in the brain called cerebral cavernous malformations (CCMs), their precise molecular cause has never been determined. These abnormal growths can lead to seizures, strokes, hemorrhages and other serious conditions.
Cardiovascular scientists at the Perelman School of Medicine at the University of Pennsylvania in Philadelphia have studied this pathway in heart development. They have discovered a set of molecular signals, triggered by CCM-linked gene defects, that could be targeted to treat the disorder.
"We hope that these findings will lead to a better understanding of the origins of CCM, and thus to treatment possibilities," said Mark L. Kahn, MD, a professor of cardiovascular medicine and senior author of the new study, published in Developmental Cell.
Although CCM has a relatively high prevalence of 1 in 200 people worldwide, it typically goes undiagnosed until symptoms are detected. Then it can only be treated with brain surgery.
Because of the difficulty of recreating the disease in lab animals, research on CCM has been slow. About 20 percent of CCM patients have a highly aggressive, inherited form of the disorder caused by inactivating one of three genes whose protein products normally work together in a complex. When knockout mice are bred without a full set of those genes, they fail to mature to have CCMs in their brains. Instead, they die in the womb, having failed to develop a working vascular system.
"Those animals die so early in their development that you just don't get enough information about what the genes normally should be doing," Kahn says.
Kahn's studies show that CCM gene knockouts remain lethal to fetal mice even when they are limited to the endothelial cells that line blood vessels and the heart. In the new study, Kahn and colleagues used advanced techniques to restrict CCM gene disruption to the endothelial cells of the developing heart, leaving the mouse vascular system to develop otherwise normally.
While the resulting mice still died before birth, this time from a failure of normal heart development not seen in human CCM patients, they survived in the womb about a week longer than standard CCM knockout mice. That enabled Kahn's team to learn more about the effects of the gene disruptions and find a previously unknown CCM-related signaling pathway.
An early clue was that the mice developed an abnormally thin layer of heart muscle. A substance called "cardiac jelly" that should separate the developing heart muscle from the heart endothelium in healthy mice was severely reduced. Analyses showed that the CCM-disrupted heart endothelial cells were overproducing protease enzymes that degrade cardiac jelly. By suppressing the activity of the protease genes, the researchers could prevent the heart defects.
The researchers traced the triggers of the protease gene overactivity to a signaling protein called MEKK3, which helps drive cell growth and survival and has been implicated in cancers. By reducing MEKK3 activity, they could shut down the jelly-degrading protease genes and prevent the heart defects. They also demonstrated that MEKK3 would bind to the CCM-complex proteins, whose absence causes familial CCM disease.
Previously, researchers noted that MEKK3 somehow associated with one of the CCM-complex proteins. "That observation didn't really lead anywhere because at the time our understanding of the CCM pathway was minimal," said Kahn. Now it seems that, at least in the endothelial cells of the developing heart, CCM proteins normally bind MEKK3. When they are not present, MEKK3 becomes abnormally-and harmfully-active.
Kahn and his team are now addressing the question of whether overactive MEKK3 also contributes to the brain vascular malformations of CCM disease by studying MEKK3 activity in a recently developed CCM mouse model. In this model, the deletion of CCM genes in endothelial cells shortly after birth produces vascular lesions in the brain that closely resemble those of human CCMs.
"If we can show that MEKK3 signaling leads to CCMs in that model, then it will be time to think about what the best therapeutic strategy will be," he said.
The co-lead authors on the paper, graduate student Zinan Zhou and medical student David Rawnsley PhD, performed most of the experiments in the study. The co-senior author was Xiangjian Zheng, a postdoctoral researcher in the Kahn Laboratory. Funding was provided by the National Heart, Lung and Blood Institute and the American Heart Association (11SDG7430025).